Precision translation stages, such as those used in IC wafer steppers, employ laser interferometers for position feedback sensors. In a typical application, three degrees of freedom are measured. Two of these are linear displacement of the stage along the X and Y axes. The third is yaw, rotation of the stage about the Z axis orthogonal to the X--Y plane.
Plane mirror interferometers are typically used to make these measurements. Two measurement mirrors are mounted on the stage, one orthogonal to the X axis, the other orthogonal to the Y axis. The length of these plane mirrors is determined primarily by the distance the stage travels in both X and Y.
Conventionally, a separate plane mirror interferometer optical assembly is necessary to measure each degree of freedom. Two optical assemblies are used to measure X and Y position. A third optical assembly is necessary on either the X or Y axis to measure yaw angle. The third interferometer measures a separate position measurement, e.g., X' at a distance d from the X position measurement. With X, X', and d (the distance between X and X'), the yaw angle theta can be calculated. EQU Theta=Arctan((X--X')/d) (1)
In addition to these typical measurements, measuring pitch and roll of the stage (rotation about the X and Y axes respectively) will be desirable as the need increases to position wafer stepper stages more precisely. With a conventional system, an additional plane mirror interferometer optical assembly is needed for each additional degree of freedom measured.
There are a number of disadvantages to using additional interferometer optical assembliess to measure each additional degree of freedom. One disadvantage is the cost of additional interferometer and associated mounting hardware and additional beam directing optics and their mounting hardware. A second disadvantage is the labor needed to align the additional interferometer and associated beam directing optics.
Other disadvantages affect the stage. The measurement plane mirror must be lengthened by the distance d, or another mirror must be added, to accommodate additional measurements on an axis. To measure pitch or roll the mirror must be taller as well as longer. With conventional plane mirror interferometer optical assemblies, d is a minimum of 2.5" because of the size of the interferometers. The size and mass of the larger mirror on the stage is undesirable because it increases the inertia of the stage. The larger mirrors can also obstruct air flow over the stage.
Additional interferometer optical assemblies also present obstructions to air flow in the stage area, in conflict with another need of precision stages in wafer steppers. To minimize temperature and pressure gradients around the stage, manufacturers are starting to use laminar air flow across the stage. The space taken up by the additional interferometers around the stage causes turbulence in the air flow over the stage, disrupting laminar flow and allowing hot spots and pressure variations to form. Thus, it is desirable to limit the number of inteferometer optical assemblies around the stage.